Defibrillation of a heart experiencing ventricular fibrillation (VF) or ventricular tachycardia (VT) occurs by applying a defibrillation shock waveform (DSW) to the heart which presents a large enough voltage gradient across the heart to stop the VF or VT. Such a voltage gradient not only enables the cessation of ventricular fibrillation, but generally re-establishes normal heart rhythm.
Currently, the medical device industry provides implantable cardiac defibrillators (ICDs) from a variety of manufacturers including Medtronic, Inc. and St. Jude Medical, Inc. For example, St. Jude Medical markets ICDs identified by the Photon®, Epic™, Epic™+ and Contour® families. Such examples may be found at the St. Jude Medical website (e.g., www.sjm.com), the information of which, pertaining to such devices, is herein incorporated by reference in its entirety (see also, http://en.wikipedia.org/wiki/Implantable_cardioverter-defibrillator, page was last modified 00:17, 18 Jun. 2006, the entire disclosure of which is herein incorporated by reference in its entirety).
The defibrillation shock waveform (DSW) for such ICDs typically comprises a biphasic waveform pulse, an example of which is illustrated in FIG. 2. As shown, such a biphasic waveform pulse generally comprises two portions: a first positive phase and a second negative phase (though the polarities may be reversed). The “tilt” of the waveform comprises the slope of the first phase and is a function of the duration of that portion of the DSW. The tilt may be viewed as the slope of the difference in voltage of the leading edge compared with the trailing edge of each pulse. Accordingly, as the duration of the first phase increases, for a given impedance, so does the tilt value. Generally, in such ICDs, the tilt or time period for each phase of the biphasic waveform pulse can be adjusted to optimize the DSW for a particular patient. The pulse width duration(s) may be fixed, yielding varying tilts depending on the impedance of the system. Alternatively, the tilt may be fixed, resulting in varying pulse widths depending on the system impedance.
Realizing a “large enough” voltage gradient or threshold to overcome VF using a DSW is dependent upon a number of factors including capacitor size of the defibrillator, maximum voltage, voltage duration of the defibrillator waveform, corresponding shape of the defibrillator waveform, and the arrangement and/or orientation of defibrillation electrodes. Furthermore, characteristics of the cardiac tissue and defibrillator system (ICD) impedance can also play an important role in determining the defibrillation threshold.
The defibrillation threshold may be minimized upon an accurate (i.e., measured) membrane time constant (or chronaxie) of a particular patient's heart. The membrane time constant is a measure of the time it takes for the membrane voltage to reach a new value, and is independent of the strength (voltage, energy or amps) of the shock of the DSW. However, the membrane time constant of the cardiac tissue is not known with precision and cannot be currently measured in vivo.
The impedance of the ICD shock is readily measured and may also influence the time course of the voltage across the cardiac tissue after the start of the shock (i.e., large voltage across the heart). Accordingly, as impedance increases, so does the time for the cardiac tissue to reach maximum value. In addition, the ideal shock duration of the DSW, or other DSW properties, may change as impedance changes.
In defibrillation, one aim is, with a shock of one polarity, to depolarize substantially all the cardiac tissue cells simultaneously or prolong refractoriness, and then remove the charge with a shock of the opposite polarity. However, when longer pulse widths are unnecessarily applied, re-initiation of fibrillation may occur after defibrillation. If a long enough pulse width is not applied, the tissue may not be simultaneously depolarized or have refractoriness prolonged, and fibrillation may persist.